When incubated with anaerobic bacteria the radioactivity from C14-metronidazole is found to bind to the macromolecules of the bacteria. The nature of this binding is being investigated by selective degradation of the radioactive material by means of peptidases, RNAases, and DNAases. The fragments that are liberated from TCA-precipitable radioactivity are being characterized in order to learn the nature of the binding of metronidazole to the macromolecule. In addition the model of the Ames mutants associated in the gastrointestinal tract of otherwise germfree rats is being investigated to determine which strains of salmonella are most stable during the association and which are most likely to give a reproducible response to a variety of carcinogens. In order to determine the site at which the revertants of the Ames mutants are formed additional strains of the intestinal microflora are being added which displace the Ames mutants from certain sites in the gastrointestinal tract. The effect of this displacement on the revertant response enables one to infer the site at which revertants were occurring.